1. Field of the Invention
The present invention relates to techniques for deriving and evaluating information within a medium. More particularly, the present invention relates to a system and method for monitoring water content or other dielectric influences in a medium such as a bed of solids.
2. Relevant Technology
Many techniques have been developed for monitoring water content or other dielectric influences in various materials and locations. Some of these techniques for measuring local water content are based on resistance, capacitance, or impedance. One technique is known as time domain reflectometry (TDR), which measures the average water content over a relatively short probe length. In TDR, two conducting probes are disposed in a medium to be measured such as soil or air. One of the probes carries a signal, while the other probe acts as a ground. A short pulse is transmitted down one of the probes by a signal generating device. The pulse is reflected off the end of the transmit probe and goes back along the same probe. When more water is present in the medium, the signal will travel more slowly, and by determining the travel time of the pulse down the probe and back to a measuring instrument, the water content averaged over the length of the probe is measured. A problem that occurs with TDR is that the water in the medium also absorbs energy from the pulse, thereby making the pulse weaker. This in turn adversely affects the accuracy of the measurement, resulting in limits on the length over which the water content can be measured.
A dielectric imaging system is disclosed in U.S. Pat. No. 5,363,050 to Guo et al., in which a transmitter transmits microwaves toward a target, and the target scatters the microwaves. The scattered waves are received by an antenna and are converted into suitable data for application to a digital computer. The computer processes the data using either a scattering matrix algorithm or a Fourier transform algorithm. The computer then generates data representative of a three-dimensional profile of dielectric permittivity which can be displayed on a display device. A problem with this system, however, is that the targeted sample for measurement must fit inside of the transmitter/antenna configuration, limiting the size of the measured sample.
In U.S. Pat. No. 4,755,944 to Glass, a method for obtaining dielectric constant and conductivity information on a subsoil structure is disclosed. In the method, at least two boreholes are created in a subsoil area to be examined, with at least one electromagnetic radiation transmitter placed in one borehole and at least one receiver placed in a second borehole, both at various locations along the boreholes. The transmitter produces a continuous constant signal which traverses the plane between the transmitter and the receiver. After measuring simultaneously both the amplitude and phase of the received electromagnetic signal, the signal information is processed using a linear approximation algorithm. Upon comparison of the processed data with standard data for nearby geological formations, it is possible to accurately determine both the dielectric constant and the conductivity of the subsoil measured. A problem with this method is that it requires measurements to be taken at multiple places along the boreholes, resulting in a labor intensive data processing method and a large set of data that must be processed to obtain the desired measurements.
A commercially available technology exists for monitoring the internal dielectric properties of transmission lines and other electrical systems. Known as network analyzers, these instruments supply electromagnetic signals at a variety of frequencies into a transmission line system. By analyzing the amount of signal that is reflected back into the instrument or that passes through the system to be returned to the instrument through a different transmission line, a network analyzer can determine the location of defects in the transmission line. The analysis that is done is some form of an inverse fast Fourier transform. Network analyzers designed to monitor the performance and integrity of such things as analyzers are designed to monitor the performance and integrity of such things as antenna wiring and undersea telephone cables where physical inspection may be difficult. However, they do not by themselves provide information about the medium around the transmission lines.
One area in which monitoring of water content is important is in biofiltering systems to decontaminate air streams. Certain biofilters work by passing a continuous flow of contaminated air through a filtering material such as compost or other organic material containing bacteria. This filtering material works only to the extent that a certain level of moisture is maintained. In the past, dielectric sensors have been placed into biofilters to give a "point" reading of moisture at a particular location. Alternatively, the total weight of the bed can be measure. The difficulty with these approaches is that water is lost in a greater amount near the influent of the air and to a lesser amount at the effluent. Thus, data related to the distribution of water composition in a biofilter, which is important to know for many types of biofilters, is not accurately obtained.
Accordingly, there is a need for an improved system and method for monitoring water content or other detectable properties that overcomes or avoids the above problems.